Stormwater Dune Infiltration Systems for Reducing Pollutant Loadings From Ocean Outfalls

The Beaches Environmental Assessment and Coastal Health Act of 2000 (BEACH Act) requires states to monitor bacteria levels in recreational coastal waters. Increased levels of bacteria increase the potential for many illnesses to beach goers, so coastal towns are forced to post advisories or close beaches after many rainfall events, which potentially decrease tourism profits. Stormwater outfalls, common in many coastal towns, empty stormwater and associated bacteria and other pollutants into the ocean or sounds. The NC Department of Transportation and the Town of Kure Beach wanted to reduce the amount of stormwater entering their ocean recreational areas. Two stormwater Dune Infiltration Systems were designed to divert a portion of the flow into the beach dunes. Sand filters have historically been successful in bacterial removal. The infiltration systems were constructed using commercially available open-bottomed infiltration chambers. Due to limited land area, the systems were designed to infiltrate 1.3 cm storms, which comprise approximately 80% of the rainfall events at the site. The watersheds of both sites were small (1.9 ha and 3.2 ha) and of mixed urban and residential land use. Water table measurements indicated a tidal influence, but approximately 2 m of sand was available for infiltration in the vertical direction. Since the installation of the system, the Dune Infiltration System has captured all the runoff produced from storm events less than the design standards of 1.3 cm (0.5 inches) per hour. The amount of fecal coliform entering the systems from these storms ranged from 358-3800 CFU/100 ml from Site L and 760-17200 CFU/100 ml for Site M. The amount of enterococcus that entered the systems from these storms ranged from 334-1653 CFU/100 ml from Site L and 1445->2005 CFU/100 ml for Site M. The groundwater at Site L had fecal coliform levels at 1 CFU/100 ml or less and enterococcus levels of 60 CFU/100 ml or less, while at Site M fecal coliform and enterococcus levels were 8 CFU/100 ml or less and 31CFU/100 ml or less respectively. Results such as these will be used to determine the feasibility of this innovative BMP in decreasing the amount of beach closures and advisories.